My first inclination here is to suggest being very down-to-earth and casual about it with him. If you seem uncomfortable or embarrassed about it, he will be, too.
Also, people have different values systems that need to be taken into account when raising your children, particularly when it comes to education about sex.
Now, I have a couple of friends with a 12 year old daughter. They have raised her as a very independent spirit and they are very open-minded and upfront people. Their daughter has known about sex for a very long time, as they have always answered her questions as accurately and openly as possible. They offer her information on topics that would make most adults cringe. For example, she's aware of what masturbation is. They let her have time to grow up. As a result, they have a very informed daughter who makes very well-thought-out decisions and is very responsible and has integrity. Hard to imagine for a child her age, but she has truly impressed me.
With boys it's different than with girls. Talk to your husband and figure out what information you want to share with your son. You may also talk to each other about how you found out about sex and what myths you were told when you were little. You can use all of this information when talking to your child. You can strike up the conversation yourselves when it's convenient by offering some of the silly things you remember being told as kids about sex or tell him how you found out... or what you heard from friends. It will open the floor to the conversation and put him at ease because you will be trading stories, not giving him a big, uncomfortable talk about sex. Once you have established open communication, you can proceed to laugh over the silly things you heard and then tell him what the truth is. Use clinical terms, if you need to, or research different methods for talking about it. ...Or, like another mom suggested, then you can suggest going to Huntsville Hospital to find out what the real truth is. After the walk-through, on the way home, ask him questions -- "what was the most interesting, weird, unexpected, gross thing they taught you?" Using "boy" terminology will bring the conversation down to his level and make him feel comfortable. This way, he sees that he can talk to his parents -- you guys become more regular people to him where sex is concerned and he gets the facts from a reliable source all at the same time. Win-win situation!